You can use NiFi Expression Language to reference FlowFile attributes, compare them to
other values, and manipulate their values when you are creating and configuring
dataflows.

In addition to using FlowFile attributes, system properties, and environment properties
within Express Language, you can also define custom properties for Expression Language use.
Defining custom properties gives you more flexibility in handling and processing dataflows.
You can also create custom properties for connection, server, and service properties, for
easier dataflow configuration.

To create custom properties for use with Expression Language, identify one or more sets
of key/value pairs, and give them to your system administrator.

NiFi properties have resolution precedence of which you should be aware when creating
custom properties:

Processor-specific attributes

FlowFile properties

FlowFile attributes

From variable registry:

User defined properties (custom properties)

System properties

Operating System environment variables

When you are creating custom properties, ensure that each custom property contains a
distinct property value, so that it is not overridden by existing environment properties,
system properties, or FlowFile attributes. Once you have added the new custom properties,
ensure that you have updated the nifi.variable.registry.properties field in the
nifi.properties file, with the custom properties location.